1. Field of the Invention
The present invention relates generally to structures for supporting an underlying particle mass such as an earthen embankment or the like. The embankments may be materials other than soil or earth. This invention relates to the concepts of mechanically stabilized particle masses. The present invention relates to an improved construction for reinforcing elements used in forming retaining walls and earthen slopes. More particularly, the present invention may be described as a reinforced earthen structure wherein reinforcement is configured so as to utilize the earth friction and/or the passive resistance between the particles and the reinforcing element. The configuration also decreases the relative stiffness of the reinforcing element making the design of such structures require less applied load to the reinforcing member. In yet another aspect of the invention the structure is characterized by reinforcing members having an alloy coating to improve their resistance to corrosion and thus increase the useful life of the reinforced structure.
2. Background of the Invention
Retaining wall structures utilizing a plurality of individual facing elements are well known. Conventionally, such facing elements are connected to the underlying mass by means of tiebacks which generally take the form of straps of various material such as metals, glass, polymers, or of a webbed sheet of similar material. In the case of sloped earth masses the facing element may be omitted.
In U.S. Pat. No. 3,686,873, Vidal discloses a new constructional work now known as a mechanically stabilized earth structure. The referenced patent also disclosed methods for construction of mechanically stabilized earth structures such as retaining walls, embankment walls, platforms, foundations, etc. In a typical Vidal construction, particulate earthen material interacts with longitudinal elements such as elongated steel strips positioned at appropriately spaced intervals in the earthen material. The elements are generally arrayed for attachment to reinforced pre-cast concrete wall panels and, the combination forms a cohesive embankment and wall construction. The elements, which extend into the earthen works, interact with compacted soil particles principally by frictional interaction and thus act to mechanically stabilize the earthen work. The elements may also perform a tie-back or anchor function.
Various embodiments of the Vidal development have been commercially available under various trademarks including the trademarks, REINFORCED EARTH embankments and RETAINED EARTH embankments. Moreover, other constructional works of this general nature have been developed. By way of example, Hilfiker in U.S. Pat. No. 4,324,508 discloses a retaining wall comprised of elongated panel members with wire grid mats attached to the backside of the panel members projecting into an earthen mass. Vidal, Hilfiker and others generally disclose large precast, reinforced concrete wall panel members cooperative with strips, mats, etc. to provide a mechanically stabilized earth construction.
Vidal, Hilfiker and others also disclose or use various shapes of wall panel members. It is also noted that in Vidal and Hilfiker the elements interactive with the compacted earth or particulate behind the wall panels or blocks, are typically rigid steel strips or mats and rely upon friction and/or anchoring interaction, although ultimately all interaction between such elements and the earth or particulate is dependent upon friction.
Federal Highway Administration's Publication No. FHWA RD-89-043, 11/1989, has lead to the development of national design codes, such as AASHTO T15 Technical Working Group MSE Retaining Wall Design Guidelines (Draft), 3/95, the American Association of State Highway Official's Specification for Bridge Design, (1994-2001), and the Federal Highway Administration's Publication No. FHWA NHI-00-043. These codes consider the relative stiffness of the reinforcement element to the surrounding particles in assessing the total load in the element. The higher the relative stiffness the more load is applied to the element. For example, at this time it is considered that straight wire mesh has to carry 2.5 times the load as would a polymer reinforcement in the same structure. The obvious disadvantage of these new design codes is an increase in cost of the inextensible reinforcing members. There is therefore a great need to manufacture metallic reinforcement members that exhibit lower stiffness ratios than plain linear elements to reduce the cost.
U.S. Pat. No. 3,686,873 discloses elongated reinforcing elements which have a substantially uniform cross section. The adjacent particles to the elements engage the surfaces of the reinforcing elements with sufficient friction to prevent displacement of the reinforcement elements in the mass.
Attempts have been made to increase the restraining forces between the particles and the reinforcement elements. For example U.S. Pat. No. 4,116,010 shows a geometry that includes hot rolling plate steel with transverse ribs on both sides. These transverse ribs entrap the surrounding particles increasing the apparent frictional forces between the particles and the elements.
U.S. Pat. No. 4,343,572 indicates a zigzag geometry but only locates it adjacent the facing to allow for settlement and earthquake loads. There is no attempt to make the entire length zigzagged to make the element extensible in nature.
In Earth Reinforcement and Soil Structures, Jones shows the many ways different people have distorted the particle end of reinforcing element which act as abutments or anchors in the particulate. Simply anchoring the end of a reinforcing element makes the structure a totally different type of design and does not qualify as a reinforced earth structure nor does it behave in a manor that allows reduction of imposed design loads because of greater extensibility. In fact, as discussed in Federal Highway Administration's Publication No. FHWA NHI-00-043, 3/2001, it actually increases the predicted load in the reinforcing element. The devices disclosed in U.S. Pat. No. 4,407,611 fall into this category.
In U.S. Pat. No. 5,525,014, I disclose a method of making linear metallic reinforcement less stiff by placing a series of yielding connections along the entire length of the reinforcing element. This method of reducing the elements relative stiffness has proven to be relatively costly.
Another concern for the reinforcement element has been the design life expectancy. Metallic reinforcement is typically coated with zinc to give some additional life span to the elements. The previous mentioned design codes allow 16 years of additional life for this type of coating. In addition the surrounding material has to meet certain electrochemical properties to assure the predicted corrosion rates. Typically, this surrounding material has to be imported to the job from rock quarries at significantly more cost than using on site materials.
Metal materials instead of zinc coated carbon steel have been tried. Stainless steels featuring a chromium content were tried, but were unsuccessful (J. M. Jailloux, “Durability of Materials in Soil Reinforcement Application”, 9th European Congress on Corrosion). Corrosion was localized significantly reducing the mechanical resistance of the reinforcing element unlike the generalized corrosion attack, as is normal with zinc coated carbon steel. Therefore the use of stainless steels was quickly abandoned.
In 1985 the Georgia Department of Transportation tried to use aluminum reinforcing elements to extend the life of one of its structures. In their Special Research Study No. 8405, “Reinforced Earth Wall Strip Serviceability Study”, they show this was not successful.
U.S. Pat. No. 4,836,718 shows how to prolong the life of metallic elements by surrounding them with a cementous material, an effective but very costly method. In U.S. Pat. No. 5,169,266 Sala discloses another very expensive way to extend the design life of the reinforcing elements beyond that of the standard galvanizied steel.
For all the above reasons corrosion of reinforcement elements in such structures represents a considerable problem in terms of the requirements of soil characteristics, and/or the cost of the anti corrosion inclusions on the reinforcing element.
AK Steel has developed an economical aluminum coating for corrugated steel pipe that addresses all the corrosion issues previously described. The performance of this material is descibed in their literature, Aluminized Steel Type 2 Corrugated Steel, 5/1999 and Aluminized Steel Type 2 Corrugated Steel Pipe Durability Update: 1995, 2/1996. Although this technology has been available since 1952, it has not been obvious to apply this same technology to particulate mass reinforcing elements.
The present invention intends to use Aluminized Type 2 Corrugated Steel coatings on the reinforcing elements to increase life expectancy and/or allow for the use of fill material with a wider range of electro-chemical properties.